The present invention addresses the shortage of suitable biomaterials and scaffolds for soft tissue engineering. A specific example of the present invention relates to tissue engineering of small diameter blood vessels (SDBV).
Tissue engineering, or regenerating fully functional and viable tissue grafts, is conventionally limited to tissue thickness less than 1 mm thick. Regenerating vascularized tissue remains elusive as well. Tissue engineering faces many challenges, most of them related to biocompatibility. Biocompatibility comprises factors such as material, chemical, and mechanical compatibility with the host. Limitations in cell sources, cell availability, also compounds the challenges of tissue engineering. Since the National Science Foundation coined the term “tissue engineering” in 1987, creating suitable biomaterials and appropriate scaffold engineering designs have been an ongoing endeavor.
The common tissue engineering model is to seed cells, autologous adult cells, stem cells, or progenitor cells into biodegradable scaffolds. The biodegradable scaffold acts as a temporary template for diseased tissues. The cell-seeded construct is then implanted to the target site in the body for tissue regeneration. The plan is that cells produce their own matrix, and new tissues form while the scaffold is gradually absorbed.
Biodegradable polymers with elastomeric properties have recently received attention for their potential use in the engineering of soft tissues such as blood vessel, heart valves, cartilage, tendon, and bladder, which exhibit elastic properties.
Vascular disease and the need for a suitable replacement of diseased tissue remains a challenge, with atherosclerotic vascular disease being the leading cause of mortality in the United States. (A report from the American Heart Association statistics committee and stroke statistics subcommittee. Heart Disease and Stroke Statistics—2006 update, 2006) . Coronary artery and peripheral vascular, SDBVs, of less than 6 mm diameter, frequently require bypass grafts. Autogenous veins, expanded polytetrafluoro-ethylene (ePTFE), and Dacron bypass grafts have been used for replacing diseased blood vessels. However, a large number of bypass grafts fail postoperatively. Acute thrombosis occurs in the early postoperative period or intimal hyperplasia (IH) occurs within months or years. Hence, multiple surgical or coronary angioplasty procedures are often required.
Tissue engineering may be able to provide a biocompatible graft with long term patency. However, tissue engineered SDBV have not attained long term patency to date.
The challenges to SDBV tissue engineering and corresponding biocompatibility are many. A suitable biomaterial needs to perform mechanical graft functions such as support for the vascular cells, a temporary extra-cellular matrix. The desired biomaterial performs analogous to a native blood vessel, having suitable strength, elasticity, and compliance. The scaffold should permit even cell distribution and nutrient delivery at matrix depths greater than 300 μm.
Compliance mismatch and or lack of strength can lead to thrombosis and/or significant inflammatory responses. Early efforts on vascular tissue engineering focused on using biodegradable synthetic polymer, such as polylactide (PLA), polyglycolide (PGA), and polycaprolactone (PCL) and their copolymers. These polymers were rolled into tubes and implanted in vivo but did not yield favorable results due to mismatch in mechanical properties and inflammatory responses.
Natural polymers, such as collagen, hyaluronic acid, chitosan, and fibrin often lack the strength and elasticity needed for vascular graft materials. (Neider et al., Biomaterials, 2002, 23(17): p. 3717-3731; Boccafoschi et al., Biomaterials, 2005, 26(35): p. 7410-7417; Remuzzi et al., Tissue Eng., 2004. 10(5-6): p. 699-710; 37. Zhang et al., J Biomed Mater Res A, 2006, 77A(2): p. 277-284). Studies using natural polymers have been unsuccessful and none of the tubes made from the natural materials above can fully represent the anatomical structure of blood vessels. Some studies have explored the use of two dimensional cell sheets to regenerate tissue, for example, SDBV. However, these tissue constructs require months of in vitro maturation and one study failed to maintain patency in vivo for more than 7 days. (L'Heureux et al., Faseb J, 1998. 12(1): p. 47-56.)
Polyurethanes are a family of polymers which have the requisite strength, elasticity and other mechanical properties to serve as functional vascular grafts. However, some of the limiting factors for urethanes is their tendency to produce creep under cyclic deformation, their poor cell compatibility and their long degradation times.
Crosslinked polyesters, like polydiol citrates have been shown to demonstrate good tolerance to creep, excellent cell compatibility and hemocompatibility and controllable degradation times. However, polydiol citrates don't have the necessary elasticity and strength to be suturable, which is one of the primary requirements of a vascular graft material
Good scaffold candidates for engineering SDBV would have good biocompatibility, which includes hemocompatibility and biodegradability. Potential scaffolds would be cell, tissue, and blood compatible. Vascular cells seeded in the scaffold would yield collagen and elastin in quantities similar to those of the native vessels. Ideal scaffolds should be non-thrombogenic and conducive to continuous endothelium layer formation. The degradation rate of the scaffolds should approach the tissue growth or remodeling rate.
It is desirable for a scaffold to have architecture which is similar to the native blood vessel. The scaffolds should compartmentalize and support fibroblasts, smooth muscle cells (SMCs), and endothelial cells (ECs). Cells should adhere to the scaffold, grow, and differentiate. The scaffold should afford communication of the cells within and across cell types.
Scaffolds should provide mechanical biocompatibility, soft and elastic, similar to the native blood vessel. Elasticity is desired to avoid compliance mismatch which was believed to contribute intimal hyperplasia, a major reason for graft failure. (Teebken et al., Eur. J. Vasc. Endovasc., 2002. 23(6): p. 475-48518; 58. He et al., Tissue Eng, 2002. 8(2): p. 213-224; Lemson et al., Eur. J. Vasc. Endovasc., 2000. 19(4): p. 336-350.) They should be able to withstand cyclic deformation without irritation to the surrounding tissues. (Wang et al., Nat. Biotechnol., 2002. 20(6): p. 602-606.)
Even when scaffolds are used for other soft tissue generation, they should permit rapid angiogenesis provide a blood supply by which nutrients are supplied to the repair site and through which waste is removed, it also provides direct access of the graft to the host's immune system to limit infection. (Hodde J., Tissue Eng, 2002. 8(2): p. 295-308.) Soft scaffolds should promote scaffold-tissue adaptation without significant mechanical irritation to the hosting tissues. Soft scaffolds should also facilitate scaffold-assembling into various shapes through, for example, folding, rolling trimming, and bending. For in vivo tissue engineering, the scaffolds should be functional for immediate implantation.
More recently, the use of soft and elastic biodegradable crosslinked poly(diol citrates) elastomers with a biphasic scaffold design showed promise for SDBV applications. However, the low molecular weight of these polymers raises processing concerns. And further, the outermost adventia layer is absent these SDBVs and suturability of the scaffold is lacking. (Yang et al., Tissue Engr., 2005, 11(11-12): p 1876-1886). Linear polyurethanes possess strong mechanical properties depending on the composition of their hard and soft segment. However, the degradability, biocompatibility, and susceptibility to permanent creep of polyurethanes still challenge their use for vascular regeneration.
Promising scaffold candidate material will be somewhat porous. It has been shown that ECs and SMCs can influence each other via heterocellular junctions and the signaling molecules secreted by both cell types. In turn, porosity which allows transfer of bio-macromolecules is desirable.